Cellulose, a polysaccharide consisting of β(1, 4)-linked glucose, formed by natural processes, (Applied Fiber Science, F. Happey, Ed., Chapter 8, E. Atkins, Academic Press, New York, 1979) has become the preeminent fiber for use in manufactured textiles, films and resins. Cotton, an especially pure form of naturally occurring cellulose, is well-known for its beneficial attributes in textile applications.
Cellulosic fibers such as cotton and rayon increasingly present sustainability issues with respect to land use and environmental imprint. This may be a significant factor leading to increased level of research into textiles containing polyester fiber blends with cellulosic materials and more sustainable solutions for cellulosic-derived materials. It is highly desirable, therefore, to discover other glucose-based polysaccharides for application in films, fibers and resins that can be economically produced from renewable resources. In addition such polymers offer materials that are environmentally benign throughout their entire life cycle.
Poly (α 1,3 glucan), a glucan polymer characterized by having α (1,3) glycoside linkages, has been isolated by contacting an aqueous solution of sucrose with a glycosyltransferase (gtfJ) enzyme isolated from Streptococcus salivarius (Simpson et al., Microbiology, 141: 1451-1460, 1995). Poly (α 1,3 glucan) refers to a polysaccharide composed of D-glucose monomers linked by glycosidic bonds. Films prepared from poly (α 1,3 glucan) tolerate temperatures up to 150° C. and provide an advantage over polymers obtained from β (1,4) linked polysaccharides (Ogawa et al., Fiber Differentiation Methods, 47: 353-362, 1980).
U.S. Pat. No. 7,000,000 disclosed preparation of a polysaccharide fiber comprising hexose units, wherein at least 50% of the hexose units within the polymer were linked via α (1,3) glycoside linkages using the gtfJ enzyme. The gtfJ enzyme utilizes sucrose as a substrate in a polymerization reaction producing poly (α 1,3 glucan) and fructose as end-products (Simpson et al., et al., Microbiology, 141: 1451-1460, 1995).
Production of low-cost poly (α 1,3 glucan) derived from sucrose, for commercial applications, requires a high yield process producing minimal undesirable by-products. In addition to poly (α 1,3 glucan), the other end product, fructose, is also a desirable product due to its application as a high value sweetener. However, fructose is also known to compete with glucose, acting as an acceptor in the gtf enzyme reaction thus hindering conversion of available glucose to poly (α 1,3 glucan) and limiting the final titer of poly (α 1,3 glucan) (Valdivia et al., (Ann. NY Acad. Sci. 542:390-394, 1988).
Robyt and Eklund (Bioorganic Chemistry, 11: 115-132, 1982) and Prat, D, et al., (Biotechnol. Letters, 9: 1-6, 1987) reported production of a by-product leucrose, a disaccharide of glucose and fructose with α (1,5) linkages, as well as fructose, by the dextranase enzyme of Leuconostoc mesenteroides when sucrose was used as substrate. Dextranase enzymes (E.C. 2.4.1.2) belong to glycosyltransferases family of enzymes and catalyze α (1,4) and α (1,6) type glycoside linkages.
Tetraborate or sodium tetraborate is a boron compound with the chemical formula: Na2B4O7. Tetraborate may form a compact polyion by corner sharing of oxygen atoms. The polyions may exist as discrete elements or they may share additional oxygen atoms to form structural units in long chains or three-dimensional networks. Tetraborate is known to react with suitable diol containing compounds (e.g., carbohydrates) in aqueous solution, to produce borate esters (T. Acree, Adv. Chem.; Am. Chem. Soc.: Washington, DC, pp 208-219, 1973). The suitability of a diol for reaction with tetraborate is determined by Oxygen-Oxygen bond distance (2.49 A to 2.63 A) within the diol and an Oxygen-Carbon-Carbon-Oxygen dihedral angle of less than 40°. Fructose, in the furanose form, is an excellent configuration for bond distance and dihedral angle compared to glucose or sucrose for reaction with tetraborate. Thus, the equilibrium constant for ester formation with tetraborate favors fructose over glucose or sucrose in solution (Pollak, V. and Mlynek, J.; Carbohydrate Research, 241: 279-283, 1993). This relatively strong association between fructose and tetraborate can be used to sequester this carbohydrate in a solution containing other sugars. Sequestration of fructose prevents its use as an acceptor in the dextranase reaction resulting in reduced leucrose synthesis. Prat et al., (supra) and Valdivia et al., (supra) described altering the yield of end products in a dextranase reaction by adding sodium tetraborate under strict conditions including specific concentrations of sodium tetraborate (<110 mM) and at pH<7.0. In the presence of 60 mM sodium tetraborate and at pH 7.0, the dextranase enzyme used by Prat et al., (supra) showed no activity at all.
Interaction between tetraborate or borate anions occurs with carbohydrates having a specific configuration (Pollak, and Mlynek, supra). It is not clear whether a similar interaction can occur between borate and poly (α 1,3 glucan). Furthermore, it has been shown that tetraborate dramatically reduces the activity of Leuconostoc mesenteroides dextranase, which belongs to a family of enzymes that catalyze α (1,4) and α (1,6) type glycoside linkages. It is not known if similar effects can be observed with the general class of glycosyltransferases which produce a high percentage of α (1,3) glycosyl linkages.
Commercial production of poly (α 1,3 glucan) and fructose from sucrose, using glycosyltransferases, requires development of methods to increase the yield of these products during the enzymatic reaction.